Optical amplifiers compensate for the losses in optical communication channels by increasing the signal power. Erbium-doped fiber amplifiers (EDFA) are important to optical communications and have enabled transmission of large amounts of data over long distances. Planar optical waveguide amplifiers are expected to reach widespread usage in the future as a result of the increased utilization of integrated photonics. In particular, polymer waveguide technology is expected to increase chip/board level communication capacity at low assembly cost and will have a profound impact on the growing field of silicon photonics. However, due to the restricted power budget, amplification of the optical signal can be required. Polymer waveguide amplifiers usually make use of optical pumping for operation. These amplifiers operate with a pump light, which have a shorter wavelength than the signal light. The absorption of the pump light leads to population inversion, and consequently to amplification by stimulated emission.
Along with several other important characteristics, an ideal optically-pumped waveguide amplifier should include some key characteristics. First, the optical signal should have large overlap with the gain material in the active waveguide for an efficient amplification. The modal gain is proportional to this overlap value. Second, the overlap integral between the signal and the pump modes should be large to make use of population inversion efficiently. Third, outside the amplifier, the pump has to be separated from the signal efficiently. Also, the amplifier should preferably not deteriorate the optical characteristic of the passive waveguide (e.g. propagation loss should not increase). Last, it is preferable to have high tolerance to variations of the device geometry and material characteristics compared to the nominal design specs. This reduces the cost by increasing the yield and relaxing process parameters.
State-of-the-art optically pumped waveguide amplifiers can satisfy only some of these requirements. The coupling and separation of the pump and the signal is obtained by using devices, such as interferometers or directional couplers, which do not have high tolerance because of their resonant operating conditions. Most of them make use of rare earth-doped polymers as gain material. This imposes stringent boundary conditions on design due to their narrow absorption band and the limited concentration caused by the tendency of these materials to form aggregates.
The prior art has important limitations. For example, U.S. Pat. No. 6,549,688 describes an optical amplifier design, which makes use of asymmetric Mach-Zehnder interferometers to multiplex and demultiplex the pump and the signal. Adiabatic couplers are proposed for coupling between different types of waveguides. These couplers do not separate the pump from the signal. Also, US 2004/0081415 describes a planar optical waveguide amplifier, in which the signal couples between the active and passive waveguides with the help of a directional coupler. U.S. Pat. No. 5,381,262 describes an optical waveguide amplifier, which includes partial erbium doping in the waveguide where the signal propagates. The pump light is coupled to the waveguide amplifier with the help of a directional coupler. In addition, EP 0,561,672 describes a waveguide amplifier, in which a gain region is obtained by doping and the pump signal is coupled in and out of the amplifier using directional couplers. Owing to a precise directional coupler design, coupling length for the signal wavelength is half of that for the pump wavelength. Therefore, the signal remains on a same waveguide, whereas the pump signal switches between waveguides. EP 1,030,413 describes a rare-earth-doped planar waveguide positioned on top of a passive waveguide. The rare-earth-doped waveguide is tapered, perpendicularly to the planar direction, to couple the pump and the signal between the waveguides. The coupler does not separate the pump from the signal. An external component is provided to separate the pump.